This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-24482, filed Jan. 31, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical encoder, and more particularly to an optical linear encoder.
2. Description of the Related Art
Currently, on the stage of a machine tool or a three-dimensional measuring instrument, an optical or magnetic type encoder is utilized in order to detect a displacement magnitude in a linear direction.
The optical encoder is of a non-contact type with high accuracy and high resolution, and is utilized in various fields because it is superior in electro-magnetic radiation resistance. In particular, in encoders requiring high accuracy/high resolution, the optical type forms the mainstream.
The optical encoder generally comprises a scale fixed to a displacement measurement target on a stage or the like, and a sensor head for detecting a displacement of the scale. The sensor head includes a light source portion, which emits light onto the scale, and a light detection portion for detecting the light that has passed through the scale, i.e., the light that has been transmitted through the scale or the light that has been reflected/diffracted on the scale. A quantity of movement of the scale is calculated in a signal processing circuit or the like connected to the sensor head based on a change in a signal detected by the photo-detector portion.
Usually, in the optical linear encoder, there is used either the scale including a displacement detection pattern and a reference position detection pattern that are formed on respective tracks, or the scale including a displacement detection pattern and a reference position detection pattern that are formed on a signal track. The scale to be used is determined depending on an intended use or conditions of a target device or the like to which the optical linear encoder is assembled.
According to one aspect of the present invention, the present invention is directed to a sensor head, which constitutes an optical encoder in combination with a movable scale including a first optical pattern and a second optical pattern. The sensor head of the present invention comprises a light source for projecting a coherent light beam onto a scale, a first photo-detector for detecting the light reflected by the first optical pattern, and at least a second photo-detector for detecting the light reflected by the second optical pattern. A straight line connecting the center of the first photo-detector with the center of the second photo-detector crosses both an x axis parallel to a movement direction of the scale and a y axis orthogonal to the x axis and parallel to a pattern surface of the scale. More preferably, the first photo-detector and the second photo-detector are arranged so that a projection of the first photo-detector projected onto a yz plane orthogonal to the x axis and a projection of the second photo-detector projected onto the same are apart from each other, and a projection of the first photo-detector projected onto a zx plane orthogonal to the y axis and the projection of the second photo-detector projected onto the same are apart from each other.
According to another aspect of the present invention, the present invention is directed to an optical encoder. The optical encoder of the present invention comprises a movable scale, and a sensor head for detecting movement of the scale. The scale includes a first optical pattern and a second optical pattern. The sensor head includes a light source for emitting a coherent light beam applied to the scale, a first photo-detector for detecting the light reflected by the first optical pattern, and at least a second photo-detector for detecting the light reflected by the second optical pattern. A straight line connecting the center of the first photo-detector with the center of the second photo-detector crosses both an x axis parallel to a movement direction of the scale and a y axis orthogonal to the x axis and parallel to a pattern surface of the scale. More preferably, the first photo-detector and the second photo-detector are arranged so that a projection of the first photo-detector projected onto a yz plane orthogonal to the x axis and a projection of the second photo-detector projected onto the same are apart from each other, and a projection of the first photo-detector projected onto a zx plane orthogonal to the y axis and the projection of the second photo-detector projected onto the same are apart from each other.
Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.